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GHB - General-Head Boundary Package

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GHB - General-Head Boundary Package

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GHB - General-Head Boundary Package

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Abbreviation in Name file

GHB

Purpose

The General-Head Boundary package is used to simulate head-dependent flux boundaries.  In the General-Head Boundary package the flux is always proportional to the difference in head.

Documentation

Related Packages

Head-Dependent Flux Boundary packages

General-Head Boundary Observation package

Supported in

MODFLOW-2000
MODFLOW-2005
MODFLOW-LGR
MODFLOW-CFP
MODFLOW-NWT
MODFLOW-OWHM

Other Notes

Special care must be taken when constructing the GHB package input file if the observations are used.  See discussion of "Observations at Cells Having More Than One Head-Dependent Boundary Feature Represented by the Same Package" in the GBOB input file.

Input Instructions

Input to the General-Head Boundary (GHB) Package is read from the file that has file type "GHB" in the name file. Optional variables are shown in brackets. All variables are free format if the option “FREE” is specified in the Basic Package input file; otherwise, the non-optional variables have 10-character fields and the optional variables are free format.

FOR EACH SIMULATION

Data Set 0

[#Text]

Item 0 is optional—“#” must be in column 1. Item 0 can be repeated multiple times.

Text—is a character variable (199 characters) that starts in column 2. Any characters can be included in Text. The “#” character must be in column 1. Except for the name file, lines beginning with # are restricted to these first lines of the file. Text is printed when the file is read.

Data Set 1

[ PARAMETER NPGHB MXL]

This optional item must start with the word “PARAMETER”.

NPGHB—is the number of general-head boundary parameters.

In MODFLOW-2005,-LGR, and -CFP the maximum number of allowed parametersis 2000.

MXL—is the maximum number of general-head-boundary cells that will be defined using parameters. MXL must equal or exceed the sum of NLST×N for all parameters, where N is the greater of 1 and NUMINST.

 

 

 

 

Data Set 2

MXACTB IGHBCB [Option]

MXACTB—is the maximum number of general-head boundary cells in use during any stress period. MXACTB includes cells that are defined using parameters as well as cells that are defined without using parameters.

IGHBCB—is a flag and a unit number.

If IGHBCB > 0, it is the unit number to which cell-by-cell flow terms will be written when "SAVE BUDGET" or a non-zero value for ICBCFL is specified in Output Control.
If IGHBCB = 0, cell-by-cell flow terms will not be written.
If IGHBCB < 0, boundary leakage for each GHB cell will be written to the listing file when "SAVE BUDGET" or a non-zero value for ICBCFL is specified in Output Control.

Option—is an optional list of character values.

AUXILIARY abc” or “AUX abc”—defines an auxiliary variable, named "abc", which will be read for each general-head boundary as part of Items 4 and 6. Up to 20 variables can be specified, each of which must be preceded by "AUXILIARY" or "AUX." These variables will not be used by the Ground-Water Flow Process, but they will be available for use by other processes. The auxiliary variable values will be read after the Cond variable.

Auxiliary parameters are used with MODPATH and the Ground-Water Transport process.  In MODPATH, the variable IFACE is specified with an auxiliary parameter.  In the Ground-Water Transport process, concentration is specified with an auxiliary parameter.  The option COMPACT BUDGET AUXILIARY must be used in the Output Control when IFACE is used for MODPATH.

NOPRINT—suppresses the printing of cell lists.

Repeat Items 3 and 4 for each NPGHB parameter.

Data Set 3

[PARNAM PARTYP Parval NLST [INSTANCES  NUMINST]]

If PARNAM is to be a time-varying parameter, the keyword “INSTANCES” and a value for NUMINST must be entered.

PARNAM—is the name of a parameter. This name can consist of 1 to 10 characters and is not case sensitive; that is, any combination of the same characters with different case will be equivalent.

All parameter names must be unique.

PARTYP—is the type of parameter to be defined. For the GHB Package, the only allowed parameter type is GHB, which defines values of the general-head boundary hydraulic conductance.

Parval—is the parameter value. This parameter value may be overridden by a value in the PVAL file.

NLST—is the number of head-dependent boundary cells that are included in a non-time-varying parameter or in each instance of a time-varying parameter.

INSTANCES – is an optional keyword that designates a parameter as time varying. The keyword is case-insensitive; that is, it may be entered in any combination of upper- and lower-case letters. If INSTANCES is present, it must be followed by a value for NUMINST. If INSTANCES is absent, PARNAM is non-time-varying and NUMINST should not be present.

NUMINST – is the number of instances that are included in the definition of a time-varying parameter, where each instance is a list of general-head boundary cells and associated properties. If the keyword INSTANCES is present, NUMINST must be present and must be at least 1. If the keyword INSTANCES is absent, NUMINST should not be present.

Data Set 4a

INSTNAM

Item 4a is read only if PARNAM is time-varying. NUMINST repetitions of Item 4 (parts a and b) are read. After each repetition of Item 4a, NLST repetitions of Item 4b are read.

INSTNAM – is the name of an instance associated with the parameter PARNAM specified in the corresponding Item 3. The name can consist of 1 to 10 characters and is not case sensitive. That is, any combination of the same characters with different case will be equivalent. Names entered for INSTNAM must be unique for any given parameter, but names may be reused for instances associated with different parameters.

Data Set 4b

Layer Row Column Bhead  Condfact [xyz]

NLST repetitions of Item 4b are required; they are read by module ULSTRD. (SFAC of the ULSTRD utility module applies to Condfact). The NLST repetitions of Item 4b follow each repetition of Item 4a when PARNAM is time-varying.

Layer—is the layer number of the cell affected by the head-dependent boundary.

Row—is the row number of the cell affected by the head-dependent boundary.

Column—is the column number of the cell affected by the head-dependent boundary.

Bhead—is the head on the boundary.

Condfact—is the factor used to calculate hydraulic conductance from the parameter value. The conductance is the product of Condfact and the parameter value.

See also "What is conductance?"

[xyz]—represents any auxiliary variables for a boundary that have been defined in Item 2. The auxiliary variables must be present in each repetition of Items 4 and 6 if they are defined in item 2.

 

 

FOR EACH STRESS PERIOD

Data Set 5

ITMP NP

ITMP—is a flag and a counter.

If ITMP < 0, non-parameter GHB data from the preceding stress period will be reused.
If ITMP 0, ITMP is the number of non-parameter general-head boundaries read for the current stress period.

NP—is the number of parameters in use in the current stress period.

Data Set 6

Layer Row Column Bhead  Cond [xyz]

ITMP repetitions of Item 6 are read by module ULSTRD if ITMP > 0. (SFAC of the ULSTRD utility module applies to Cond.) Item 6 is not read if ITMP is negative or 0.

Layer—is the layer number of the cell affected by the head-dependent boundary.

Row—is the row number of the cell affected by the head-dependent boundary.

Column—is the column number of the cell affected by the head-dependent boundary.

Bhead—is the head on the boundary.

Cond—is the hydraulic conductance of the interface between the aquifer cell and the boundary.

See also "What is conductance?"

[xyz]—represents any auxiliary variables for a boundary that have been defined in Item 2. The auxiliary variables must be present in each repetition of Items 4 and 6 if they are defined in item 2.

 

 

Data Set 7

[Pname [Iname]]

Item 7 is repeated NP times. It is not read if NP is negative or 0. Iname is read if Pname is a time-varying parameter.

Pname—is the name of a parameter that is being used in the current stress period. NP parameter names will be read.

Iname – is an instance name that is read only if Pname is a time-varying parameter. Multiple instances of the same time-varying parameter are not allowed in a stress period.